There has been a growing demand in thermal management systems for integrated temperature sensors with on-chip PN junction diodes/transistors, analog-to-digital converters (ADCs) and bus interfaces. For example, these so-called “smart” temperature sensors are widely used in PCs, laptops and LCD displays to monitor the system temperature. Such applications typically require low-cost temperature sensors with a desired accuracy within +/−1.0 degree Celsius (deg C.). In other words, it is desired that the inaccuracy be below +/−1.0 deg C.
In order to convert temperature to a digital value, both a well-defined temperature-dependent signal and a temperature-independent reference signal are required. The temperature-dependent signal can be obtained by comparing the difference in the base-emitter voltages of two bipolar (BJT) transistors at different current densities. The temperature-independent reference signal can be obtained by using a bandgap reference voltage, which also depends on the base-emitter voltages. The base-emitter voltage Vbe of a single transistor in its forward-active region has a typical slope of about −2 mV/deg K and an extrapolated value at 0 deg K of roughly 1.2 V. The delta Vbe between the base-emitter voltages of two such transistors biased at different collector currents is proportional to absolute temperature (ptat) and has a value and a temperature coefficient which are typically one order of magnitude smaller than those of Vbe. Equations 1 and 2 below define such relationships.Vbe=(kT/q)ln(Ic/Is)   (Eq. 1)delta Vbe=Vbe1−Vbe2=(kT/q)ln[(Ic1*Is2)/(Ic2*Is1)]=(kT/q)ln(M)   (Eq. 2)
Where,                Is the saturation current,        Ic is the collector current,        k is Boltzmann's constant,        q is the electron charge,        T is the temperature in degree Kelvin, and        M=(Ic1*Is2)/(Ic2*Is1).        
Because of the small value and the small temperature coefficient of the delta Vbe, special care has to be taken to avoid the occurrence of errors due to mismatch of bipolar transistors and op-amp offset. For example, where M=8, a mere 0.53 mV of mismatch in Vbe (or offset voltage of the op-amp used to sense the delta Vbe; the offset voltage directly added to the delta Vbe) produces a 1% error in the absolute temperature, which is translates to a 3 deg C. error at ambient temperature. Where M=3, this mismatch or offset should be smaller than 10 uV to obtain a negligible temperature error (0.1 deg C.). Previous designs have employed nested-chopping/auto zeroing to reduce temperature errors due to the effects of mismatch in Vbe and offset voltage of amplifiers used in the readout circuit.
Various temperature sensors, some of which are temperature to digital converts, are known in the prior art, including those listed below, each of which are incorporated herein by reference: K. Kujik, “A precision reference voltage source,” IEEE J. Solid State Circuits, vol. SC-8, pp. 222-226, June 1973; B. Song and P. Gray, “A precision curvature-compensated CMOS bandgap reference,” IEEE J. Solid State Circuits, vol. SC-18, pp. 634-643, December 1983; M. Tuthill, “A switched-current, switched-capacitor temperature sensor in 0.6 um CMOS,” IEEE J. Solid State Circuits, vol. SC-33, pp. 1117-1122, July, 1998; M. Pertijs, A Niederkorn, B. Mckillop, A. Baker, J. Huijsing, “A CMOS smart temperature sensor with a 3 sigma inaccuracy of +/−0.5 deg C. from −50 to 120 deg C.,” IEEE J. Solid State Circuits, vol. SC-40, pp. 454-461, February 2005; M. Pertijs, K. Makinwa, J. Huij sing, “A CMOS smart temperature sensor with a 3 sigma inaccuracy of +/−0.1 deg C. from −55 to 125 deg C.,” IEEE J. Solid State Circuits, vol. SC-40, pp. 2805-2815, December, 2005; J. Huijsing, R. J. van de Plassche and W. Sansen, “Analog Circuit Design”, Boston/Dordrecht/London: Kluwer Academic, 1996, pp. 350-351; and M. Pertijs, A. Bakker, J. Huijsing, “A High-Accuracy Temperature Sensor with Second-order Curvature Correction and Digital Bus interface”, in Proc. ISCAS, May 2001, pp. 368-371.
Nevertheless, each of the above mentioned temperature sensor have certain aspects that need improving. In other words, there is still a desire to improve upon previous temperature sensors, including previous temperature to digital converters.